Disable the internal pullups after i2c initialization
Remove Firmata. It is not used.
Refactored si570 code in an Si570 class
some cleanup in the main sketch
Automatically calibrate the Si570 library using factory settings + cleanup & better error management

In the Si570 class:

 - Use the factory-calibrated output frequency to automatically calculate the internal
   crystal frequency and use that instead of hard coded values.
 - During initialization detect errors talking to the Si570.
 - Use multi-bytes i2c reads and writes when possible (ie: read/write multiple bytes
   without re-starting a transaction). Only works for read/writes in consecutive registers.
 - Use types with explicit size where that makes sense (uint8_t instead of char, uint64_t, etc)
 - Added some debugging on the serial console through a "debug" function

In the main sketch:

 - Show a message on the LCD if the Si570 is unreachable.
 - Initialize the serial port for debugging
 - Fix some compilation warnings with string litterals being casted to char*
 - Removed unused variables
Merge pull request #4 from sarfata/refactor-si570

Refactor si570 + Automatically calibrate the module + Detect errors